News International has emailed thousands of people to warn them that one or more hackers have copied personal details of thousands of people who entered competitions and polls on the Sun's website and are posting them publicly on the internet.

In an email sent out late on Monday evening, the director of customer data for News International, Chris Duncan, says that in a hacking attack on 19 July – when a fake story claiming that Rupert Murdoch was dead was planted on the site – "some customer information from competitions and polls was breached". Among the details are names, addresses, date of birth, email and phone numbers. "No financial or password information was compromised," Duncan writes.

Samples from files in which Sun readers entered a Monarch quiz in 2009, as well as a list of Scottish students, the Miss Scotland contestants database, a Wrigleys football competition, an Xbox competition, referendum, royal wedding well-wishers, and a forum for bullied people have been posted on the Pastebin site, a popular site among hackers for posting public messages anonymously. Many include personal information including phone numbers and addresses.

Duncan says that News International is working closely with the police and the information commissioner "to ensure that all steps are taken to retrieve the files involved".

The files appear to have been released by a hacker who on Twitter calls himself Batteye. He claims not to have any association or affiliation with the hacking group LulzSec, which claimed responsibility for the NI hack on 19 July. Three Britons alleged to have been involved with LulzSec have been arrested in the past six weeks.

In what appears to be a manifesto of his motivations, Batteye wrote on Pastebin that "Mankind makes mistakes. Mankind is all the better for them. Mankind learns from them. Some people, however, do not learn. Until these people are pruned by natural selection, incarceration, or otherwise, then mankind will not develop. We will remain prey to the 'malicious' type of hacker that steals credit card information, or deletes voicemail messages and pushing the victims family to grieve more for their loved ones.

"This is unacceptable... We will begin today be presenting to you, various files obtained from the Sun, a company within the News Corp group. We will continue, then, by exposing the world for what it is; a less than perfect place where we cannot trust those who we ask to protect our information."